It is generally known that a band stop filter is configured to pass most frequencies through a bandwidth unaltered, but attenuates those in a specific range to very low levels. It is also known that a band-stop filter with a narrow stopband is a notch filter.
The present invention is directed to a coaxial tunable band stop filter utilizes tuning elements, such as PIN diodes and varactor diodes for tuning a coaxial resonator to change the resonance frequency of the resonators. A voltage is applied to the tuning elements to change their capacitance, such that they electrically lengthen and shorten the coaxial resonator. The voltage varies the center frequency of the bandwidth. When the resonators are electrically extended or shortened in length, the center frequency in the bandwidth is changed accordingly. The bandwidth for the coaxial tunable band stop filter is tunable to increase and decrease based on the position of the center frequency.
Generally, band stop filters require precise transmission characteristics to attenuate a band of frequencies at a specific bandwidth and to pass frequencies outside the bandwidth at both higher and lower frequencies. The band stop filters are generally characterized by a bandwidth, center frequency, insertion loss, selectivity or rejection, ripple and return loss. In one known parameter, the band stop filter may have a center frequency of 1000 MHz and bandwidth of 100 MHz or 950-1050 MHz.
Often, resonators have a resonance frequency that can be electronically controlled by means of varactors diodes, PIN diodes, MEMs, etc. The varactor diodes could include a tuning band extending beyond the very narrow limits usually obtainable with conventional networks.
It is known in the art that, conventional coupling network between a dielectric resonator and a varactor diode, both placed on the same face of a microstrip circuit, includes a length of 90° transmission line which is terminated at one side only, by means of the varactor diode and near to which the resonator is fixed. The control voltage is applied to the varactor diode through a suitable RF decoupling network.
In many instances, the transmission line and varactor diode assembly is dimensioned in such a way as to resonate at about the nominal frequency of the dielectric resonator. During the circuit operation, the magnetic field lines of the resonator interlink with the transmission line. By varying the bias voltage of the varactor diode, the capacitance of the latter is modified and the change of the resonance frequency of the dielectric resonator is thus determined.
Thus, an unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies in non-tunable filters. Even though the above cited methods for band stop filters meets some of the needs of the market, a coaxial tunable band stop filter that is electrically tunable through tuning elements, such as PIN diodes and varactor diodes is still desired.